The present invention relates generally to methods of controlling bipolar ionization systems and, more particularly, to a method of offset voltage control for bipolar pulse mode ionization systems.
Air ionization is the most effective method of eliminating static charges on non-conductive materials and isolated conductors. Air ionizers generate large quantities of positive and negative ions in the surrounding atmosphere which serve as mobile carriers of charge in the air. As ions flow through the air, they are attracted to oppositely charged particles and surfaces. Neutralization of electrostatically charged surfaces can be rapidly achieved through the process.
Air ionization may be performed using electrical ionizers which generate ions in a process known as corona discharge. Electrical ionizers generate air ions through this process by intensifying an electric field around a sharp point until it overcomes the dielectric strength of the surrounding air. Negative corona occurs when electrons are flowing from the electrode into the surrounding air. Positive corona occurs as a result of the flow of electrons from the air molecules into the electrode.
To achieve the maximum possible reduction in static charges from an ionizer of a given output, the ionizer must produce equal amounts of positive and negative ions. That is, the output of the ionizer must be xe2x80x9cbalanced.xe2x80x9d If the ionizer is out of balance, the isolated conductor and insulators can become charged such that the ionizer creates more problems than it solves. Ionizers may become imbalanced due to power supply drift, power supply failure of one polarity, contamination of electrodes, or degradation of electrodes. In addition, the output of an ionizer may be balanced, but the total ion output may drop below its desired level due to system component degradation.
A charge plate monitor is typically used to calibrate and periodically measure the actual balance of an electrical ionizer, since the actual balance in the work space may be different from the balance detected by the ionizer""s sensor. The charge plate monitor is also used to periodically measure static charge decay time. If the decay time is too slow or too fast, the ion output may be adjusted by increasing or decreasing the preset ion current value. This adjustment is typically performed by adjusting two trim potentiometers (one for positive ion generation and one for negative ion generation) or by adjusting a value stored in software that represents an ion current reference value. Periodic decay time measurements are necessary because actual ion output in the work space may not necessarily be the same as the expected ion output for the ion output current value set in the ionizer.
A room ionization system typically includes a plurality of electrical ionizers connected to a single controller. A conventional room ionization system may include a plurality of ceiling-mounted emitter modules (also, referred to as xe2x80x9cpodsxe2x80x9d) connected in a daisy-chain manner by signal lines to a master controller.
Traditionally, a sensor is used in conjunction with a room system or a mini environment ionizer bar to control the offset voltage generated by the ionization system steady state direct current (DC) operation. Steady state DC operation implies constant production of both polarities of ionization from independent positive and negative pins. In this case, the offset voltage is the voltage that would develop on an isolated conductor in the presence of the ionization system. A charge plate monitor is used to determine the offset voltage of the ionization system. Sensors used for this type of application attempt to have essentially infinite input impedances such that they accurately measure offset voltage for negative feedback control of offset voltage. Alternatively, the sensors sample the current produced by the ionizer. Generally, an end user is attempting to control offset voltage to within some threshold critical for the success of their particular process or processes.
Controlling the offset voltage in a given environment is becoming increasingly important. Many modern semiconductor devices/wafers and disk drive heads (giant magnetoresistive or GMR heads) and the like are susceptible to electrostatic discharge (ESD) at lower voltage potentials. For example, such devices may be damaged by voltages around 100V so controlling to 50V or below may be of interest to avoid product losses and malfunctions.
Pulsing systems offer good charge decay times, which are the measure of rate of charge neutralization, and are useful in environments with poor or inadequate airflow. However, most prior art pulsing systems do not attempt to limit offset voltage during pulse mode operation. As a result, pulse times and output levels must carefully be selected to achieve the desired charge decay time without producing excessive offset voltage swing levels. In one such prior art system shown in FIG. 1A, it is very difficult to use long pulse times as they will generate very large offset voltage swings. Offset voltage must be maintained within acceptable limits so that device damage does not occur. The objectionable offset voltage swings generated in a pulse mode system are such that during positive and negative pulses, only one polarity of ionization is provided. The resulting stream of ionization creates swings of offset voltages that can be measured on an isolated conductor. To limit the swing, the end user is forced to adjust the output of the pulse ionization system to a lower level, or select a pulse time that achieves the same result. In either case, charge decay times can become longer which is an undesirable side effect.
FIG. 1B shows that some prior art systems suggest using an xe2x80x9coff-timexe2x80x9d between pulses of alternate polarities to limit the offset voltage swing. In practice, this technique has several disadvantages. The high voltage power supplies used to provide ionization generally have long time constants associated with them that make a rapid shut down or a realized turn off difficult to attain. In the xe2x80x9coff-timexe2x80x9d technique, although the input to the high voltage supply is reduced, the output continues to produce ionization and, as a result, there is still a corresponding increase in offset voltage. Further, the duration of the xe2x80x9coff-timexe2x80x9d that the system uses also reduces the overall ion output of such a system that uses xe2x80x9coff-time.xe2x80x9d Ultimately, ionization systems are installed to produce ions so this is an obvious drawback. Thus, the technique depicted in FIG. 1B has the inherent disadvantage of producing a lower overall ion density in the environment.
What is needed, but not provided by the prior art ionization systems, is a method of controlling the offset voltage generated in pulse mode ionization within user designated limits while having charge decay times that are still adequate or better than adequate. Further, what is needed, but not provided by the prior art ionization systems, is a method of controlling a continuous ionization system in conjunction with a sensor by tracking the sensor alternately for positive and negative setpoints.
Briefly stated, the present system comprises a method of offset voltage control for pulse mode ionization systems, wherein the ionization system has positive and negative power supplies. The method includes controlling the overlap of the outputs of the positive and negative power supplies and determining an overlap that achieves a desired offset voltage. The method also includes storing the offset voltage and the corresponding overlap in memory. The method also includes controlling the duty cycle of the outputs of the positive and negative power supplies to achieve the desired offset voltage based upon the stored offset voltage comparison.
The present invention also comprises a bipolar ionization apparatus that includes a positive high voltage power supply having an output with at least one positive ion emitting electrode connected thereto and configured to generate positive ions and a negative high voltage power supply having an output with at least one negative ion emitting electrode connected thereto and configured to generate negative ions. The bipolar ionization apparatus further includes a controller that is configured to control a duty cycle of the outputs of the positive and negative high voltage power supplies to achieve a desired offset voltage by causing the outputs of the positive and negative high voltage power supplies to overlap by a selected amount of time in excess of zero.